The present invention relates to a device for detecting instantaneous voltage drops of a DC power source and, more particularly, to an improvement in an instantaneous voltage drop detector for use in a mobile radiotelephone for detecting a voltage drop of an on-board DC power source which drops ends within a predetermined period of time, i.e. an instantaneous voltage drop as will be referred to hereinafter.
A mobile radiotelephone is powered by a battery which is mounted on a motor vehicle. When a starting motor of a motor vehicle is driven to start an engine, a current of several tens of amperes flows so that the voltage of an on-board battery is temporarily lowered below a predetermined level by the internal resistance of the battery and cables. The phenomenon whereby the voltage undergoes such a drop and is restored to normal within a predetermined period of time (hereinafter referred to as instantaneous voltage drop decision time or simply decision time) such as three seconds is generally referred to as an instantaneous voltage drop.
When the voltage of a power source associated with a mobile radiotelephone drops below a predetermined level even for a short time, the radio section of the radiotelephone is rendered immediately inoperable. Therefore, when an instantaneous voltage drop occurs in an on-board battery or power source as stated above, the radio section remains inoperable even after the restoration of the source voltage. During this time the radiotelephone is out of communication with a central station, simply wasting the communication channel. In light of this, a modern mobile radiotelephone system is provided with an instantaneous voltage drop detector for detecting an instantaneous voltage drop. When an output of the detector shows that an instantaneous voltage drop has occurred, the radio section is energized to cause the radiotelephone to again communicate with the central station.
A prior art instantaneous voltage drop detector of the kind described is constructed as follows. A source voltage lowered below a predetermined level is sensed by a reset circuit. The resulting low-level output of the reset circuit is applied to one input terminal of a comparator via an integrator so as to be compared with a reference voltage which is fed to the other input terminal of the comparator. The result of comparison outputted by the comparator is used to detect the instantaneous voltage drop, i.e., a discharging time constant of the integrator defines an instantaneous voltage drop decision time. For details of such a technique, reference may be made to U.S. patent application Ser. No. 07/158,610.
However, the prior art instantaneous voltage drop detector is not capable of detecting continuously occurring instantaneous voltage drops with accuracy. More specifically, the charging and discharging time constants of the integrator installed in the prior art detector are equal to each other. Hence, when an instantaneous voltage drop is followed by another instantaneous voltage drop before the amount of electric charge discharged from the integrator due to the preceding voltage drop is charged again in the integrator, the voltage level applied to the comparator is apt to remain lower than the reference level despite the fact that the voltage drops are instantaneous.